Cathode container for sodium-sulfur cells

ABSTRACT

A corrosion resistant structure for a sodium-sulfur cell is described which comprises a container structure for the sulfur cathode including an outer metallic container having an inner surface defining a volume for containing the cathode, a layer of glass on the inner surface of the container, and a current collector of metallic foil disposed within the container adjacent the glass layer for containing the cathode. A preferred embodiment includes stainless steel as the container material and molybdenum foil as the current collector material.

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION

The present invention relates generally to rechargeable cells containingliquid sodium and sulfur and more particularly to a corrosion resistantcathode container and current collector structure for sodium-sulfurcells.

In the operation of a sodium-sulfur cell, both the sodium and sulfur areliquid at the operating temperature of the cell. Sodium ions from theliquid sodium anode are transported through a ceramic electrolyte to theliquid sulfur cathode and react with sulfur to form sodium polysulfidesaccording to the reversible reaction,

    2Na+xS⃡Na.sub.2 S.sub.x (x=3, 4, or 5).

The sodium ions surrender charge in the reaction, and the currentgenerated thereby is conducted through the sulfur and supporting wickmaterial to a metallic container which is connected to a terminal. Themetallic container is typically stainless steel with a chromium coatingon the inner surface thereof to impede corrosion of the container bysodium polysulfides formed during cell operation. The chromium coatingeventually corrodes by reaction with polysulfides to form NaCrS₂ whichseriously impairs performance of the cell, particularly throughdeterioration of the electrolyte.

The invention solves or substantially reduces in critical importance theforegoing problem with existing sodium-sulfur cell structures byproviding a reliable corrosion resistant container and current collectorstructure for the liquid sulfur cathode. The invention comprises ametallic outer container for the cathode having on the inner surfacethereof a coating of electrically insulating, nonreactive glass.Adjacent the glass coating is a tubular metallic cathode currentcollector.

It is therefore a principal object of the invention to provide animproved liquid sodium-sulfur cell structure.

It is another object of the invention to provide a corrosion resistantcathode container structure for sodium-sulfur cells.

These and other objects of the invention will become apparent as thedetailed description of representative embodiments proceeds.

SUMMARY OF THE INVENTION

In accordance with the foregoing principles and objects of theinvention, a corrosion resistant structure for a sodium-sulfur cell isdescribed which comprises a container structure for the sulfur cathodeincluding an outer metallic container having an inner surface defining avolume for containing the cathode, a layer of glass on the inner surfaceof the container, and a current collector of metallic foil disposedwithin the container adjacent the glass layer for containing thecathode. A preferred embodiment includes stainless steel as thecontainer material and molybdenum foil as the current collectormaterial.

DESCRIPTION OF THE DRAWINGS

The invention will be clearly understood from the following detaileddescription of representative embodiments thereof read in conjunctionwith the accompanying drawing wherein:

FIG. 1 is a view in axial section of a liquid sodium-sulfur cellstructure of the invention;

FIG. 2 is a fragmentary sectional view of a prior art containerstructure; and

FIG. 3 is a view along line C--C of FIG. 1.

DETAILED DESCRIPTION

Referring now to the drawings, FIG. 1 shows a view in axial section of arepresentative sodium-sulfur cell 10 of the invention. Hermeticallysealed metallic container 11 of generally tubular shape defines within afirst container portion 11a first chamber 13 for containing sodium 15(m.p. about 97.8° C.) comprising the anode of cell 10, and in a secondcontainer portion 11b, a second chamber 17 for containing sulfur 19(m.p. about 112.8° C.) comprising the cathode of cell 10. Sodium 15 andsulfur 19 are liquid at normal operating temperatures for cell 10 (about300°-350° C.). Container 11 may comprise substantially any metal oralloy known in the art for sodium-sulfur cell structure. In accordancewith the teachings below in relation to FIG. 3, the inner surface ofcontainer 11 receives a glass coating and, therefore, container 11 maycomprise any suitable metallic material such as aluminum, stainlesssteel, cold rolled steel or ceramic, although E-Brite™ stainless steelin thickness of about 0.012 to 0.030 inch may be preferred.

An electrolyte 21 comprising solid ceramic material in generally tubularshape separates chamber 13 containing sodium 15 from chamber 17containing sulfur 19. Electrolyte 21 may comprise any ceramic materialnormally included in conventional cells for conducting sodium ionsthrough the thickness (usually about 0.030 to 0.120 inch) thereof incell operation including beta alumina, beta alumina, Nasicon (acronymfor sodium superionic conductor containing sodium, zirconium, silicon,phosphorus and oxygen) or haloborate glasses. One closed end 22 ofelectrolyte 21 extends into chamber 17 whereby liquid sulfur 17 contactsand wets outer surface 23 of electrolyte 21 along a substantial portionof the length thereof, and an open end 24 communicates with chamber 13allowing liquid sodium 15 to fill the inner volume defined byelectrolyte 21 for contacting and wetting inner surface 25 thereof.Suitable conventional wicking materials (not shown) may be includedwithin chambers 13,17 for facilitating flow by capillary action ofliquid sodium 15 and liquid sulfur 19 in wetting respective inner andouter surfaces 23,25 of electrolyte 21; typical wicking materials forliquid sodium 13 may include wire mesh of stainless steel, copper,nickel or molybdenum, and for liquid sulfur 19 may include graphitefiber fabric, felt or mat. Open end 24 of electrolyte 21 is sealedwithin annular insulating block 26 of alpha alumina or other suitableceramic insulator conventional to sodium-sulfur cell structures.Metallic foil layer 27 (0.001 to 0.020 inch thick) of molybdenum, goldor other suitable metal or alloy defines chamber 17 and separates sulfur19 contained therein from contacting container 11 to prevent corrosionby sodium polysulfides formed in operation of cell 10. Layer 27 furthercomprises an electric current collector in the operation of cell 10 andis connected to or integral with metallic (e.g., like material to layer27) collector plate 29 and terminal 31 on second container portion 11bas suggested in FIG. 1. First container portion 11a containing liquidsodium 15 is electrically insulated from second container portion 11b asat insulating block 26, and may comprise a second electrical contact forcell 10.

Referring now to FIG. 2, shown therein is a cross-sectional view of aconventional container 11' structure for sodium-sulfur cells. Container11' conventionally includes a first metallic outer layer 33. Layer 33 isetched or otherwise treated and plated with layer 34 of chromium forcorrosion resistance; adherence or compatibility layer 35 compris1ngzinc, copper or other suitable metal may be applied to layer 33 prior toplating layer 34.

Referring now to FIG. 3, shown therein is an enlarged cross sectionalview along line C--C of FIG. 1. In accordance with the teachings of theinvention, current collector portion 11b containing the sulfur 19cathode includes an outer layer 37 of metal or alloy, including thegroup listed above suitable for being plated with layer 38 of glass.Glass layer 38 is selected to provide an electrically insulating,chemically nonreactive and corrosion resistant coating to container 11and may comprise a Pyrex™ or other suitable composition glasses.Functionally satisfactory glass layers 38 may be applied to a thicknessof about 0.0005 to 0.005 inch by conventional deposition techniquesknown in the applicable art field. A preferred glass material isS5210-2C (Solar Turbine Corp), a commercially available glass havingexcellent stability against chemical attack and coefficient of thermalexpansion compatible with preferred layer 37 materials. Layer 38 ofS5210-2C to about 0.002 inch thickness provides substantial resistanceto chemical attack and eliminates need for a chromium or otheradditional metallic layer (34 in FIG. 2) included in conventional cellstructures. With reference again to FIG. 1, glass layer 38 as shown inFIG. 3 covers the entirety of the inner surface of container portion 11bdefining chamber 17, and is preferably included on insulating block 26and regions of contact between block 26 and container portion 11b toavoid corrosion of any metallic seals included in structures joiningcontainer portions 11a, 11b or electrolyte 21 to block 26.

Glass layer 38 is electrically nonconductive, and therefore, metalliclayer 27 is included partly to provide a shunt for conducting current tocollector plate 29 and/or terminal 31. Molybdenum is a preferredmaterial for layer 27 since it is highly resistant to corrosivesgenerated during operation of cell 10 and can be fabricated as foil. Inthe fabrication of cell 10 in accordance with the teachings of theinvention, foil layer 27 may be formed into tube shape corresponding insize to the inner dimensions of container 11 and welded (e.g. byelectron beam welding) to collector plate 29 as at weld joint 41.Closure disk 42 of like material to layer 37 may be welded to an openend of container portion 11b for closure of cell 10. The outer surfaceof collector plate 29 and inner surface of disk 42 may be plated with aglass layer similar to that applied to container portion 11b, withterminal 31 electrically connected through plate 29 to foil layer 27.Glass layer 38 defining the inner surface of container portion 11b maybe melded with the glass layer on disk 42 by heating that end ofcontainer portion 11b.

The invention therefore teaches a corrosion resistant structure forsodium-sulfur cells. It is understood that modifications to the inventonmay be made as might occur to one with skill in the field of theinvention within the scope of the appended claims. All embodimentscontemplated hereunder which achieve the objects of the invention weretherefore not shown in complete detail. Other embodiments may bedeveloped without departing from the spirit of the invention or from thescope of the appended claims.

I claim:
 1. A container structure for the cathode of a sodium-sulfurelectric cell comprising:(a) a metallic first container having an innersurface defining a substantially closed chamber for containing sulfurcomprising the cathode of said cell; (b) a thin continuous layer ofglass on said inner surface of said metallic first container; and (c) asubstantially closed metallic foil second container disposed within saidmetallic first container for containing said sulfur comprising saidcathode of said cell and for collecting current from said cathode. 2.The structure of claim 1 wherein said metallic first container comprisesa material selected from the group consisting of stainless steel,aluminum, and cold rolled steel.
 3. The structure of claim 1 whereinsaid thin continuous layer of glass has thickness of from about 0.0005to about 0.005 inch.
 4. The structure of claim 1 wherein said mettalicfoil second container comprises a material selected from the groupconsisting of molybdenum and gold.
 5. The structure of claim 4 whereinsaid metallic foil second container has wall thickness of from about0.001 to about 0.020 inch.
 6. In a hermetically sealed sodium-sulfurelectric cell including a solid ceramic electrolyte separating a firstchamber containing a sodium anode from a second chamber containing asulfur cathode, an improvement comprising a corrosion resistantcontainer structure for said cathode including:(a) a metallic firstlayer having an inner surface defining and substantially enclosing thesecond chamber; (b) a thin continuous layer of glass on said innersurface of said metallic first layer; and (c) a substantially closedmetallic foil second layer disposed within said metallic first layer andenclosing the cathode for collecting current from said cathode.
 7. Thecell of claim 6 wherein said metallic first layer comprises a materialselected from the group consisting of stainless steel, aluminum, andcold rolled steel.
 8. The cell of claim 6 wherein said thin continuouslayer of glass has thickness of from about 0.0005 to about 0.005 inch.9. The cell of claim 6 wherein said metallic foil second layer comprisesa material selected from the group consisting of molybdenum and gold.10. The cell of claim 9 wherein said metallic foil second layer hasthickness of from about 0.001 to about 0.020 inch.